Calibration for test and measurement instrument including asynchronous time-interleaved digitizer using harmonic mixing

ABSTRACT

A test and measurement instrument includes a coefficient storage facility coupled to a programmable filter. The coefficient storage facility is configured to store at least two pre-determined filter coefficient sets, and configured to pass a selected one of the at least two pre-determined filter coefficient sets to the filter based on a measurement derived using a compensation oscillator. The measurement may include clock delay and clock skew. In some examples the test and measurement instrument may additionally adjust clock delay and/or clock skew in addition to selecting appropriate filter coefficients.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/391,637, filed Dec. 27, 2016, which issued as U.S. Pat. No.10,330,705, which is a continuation of U.S. patent application Ser. No.14/971,727, filed Dec. 16, 2015, which issued as U.S. Pat. No.9,568,503, which is a continuation-in-part of U.S. patent applicationSer. No. 14/254,373, which issued as U.S. Pat. No. 9,306,590, which is acontinuation-in-part of U.S. patent application Ser. No. 13/116,234,which issued as U.S. Pat. No. 8,742,749. This application is alsorelated to U.S. patent application Ser. No. 14/229,307, which was issuedas U.S. Pat. No. 9,432,042. The entire contents of all of these citedapplications are hereby incorporated by reference into this application.

BACKGROUND

This disclosure relates to test and measurement instruments and, moreparticularly, to calibration of test and measurement instrumentsincluding one or more asynchronous time-interleaved digitizers, whichuse harmonic mixing for reducing noise.

Useable bandwidths of test and measurement instruments, such as digitaloscilloscopes, can be limited by an analog to digital converter (ADC)used to digitize input signals. The useable bandwidth of an ADC can belimited to the lesser of the analog bandwidth or one half of a maximumsample rate of the ADC. Various techniques have been developed todigitize higher bandwidth signals with existing ADCs. One such techniqueis described in the above-referenced patent and applications, whichincludes splitting an input signal into a number of split signals eachincluding substantially all of the bandwidth of the input signal. Thenthe split signals are respectively mixed with harmonic mixers anddigitized. The digitized, split signals can be recombined to make areconstructed input signal. This technique is referred to as ATI, or anAsynchronous Time Interleaved system.

In the event of interleaving errors due to analog mismatch of such asystem, hardware adjustments can be made for mixing clock amplitude andphase. The adjustments can also be calibrated to minimize interleavemismatch spurs. Alternatively, or in addition, hardware mismatches canbe characterized, and a linear, time-varying correction filter may beused to cancel the interleave spurs.

Previously, such calibration occurred at the factory before aninstrument is shipped to a customer. Although the instruments areinitially factory calibrated, hardware performance may drift from theircalibrated state based on environmental conditions at runtime, such astemperature and humidity. Calibrating for a particular hardware state ofsuch a sensitive device, however, requires access to a signal sourcethat spans the full frequency range of the internal digitizer. Thebuilt-in calibration oscillators described in the '373 application,however, are not tunable over the entire range of the potential signalsources. Therefore, calibration of systems having a built-in calibrationoscillator that does not span the entire range of potential signalsources suffers.

Embodiments of the invention address these and other limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ADC system for a test and measurementinstrument using harmonic mixing according to an embodiment of theinvention.

FIGS. 2-8 illustrate examples of spectral components of various signalsin the ADC system for the test and measurement instrument of FIG. 1.

FIGS. 9A, 9B, and 10-12 are block diagrams of examples of harmonicmixers of FIG. 1.

FIG. 13 is an embodiment of a block diagram of the asynchronous timeinterleave (ATI) digitizer of FIG. 1 with a compensation oscillator.

FIG. 14 is another embodiment of a block diagram of the ATI digitizer ofFIG. 1 with a compensation oscillator.

FIG. 15 is a block diagram of an embodiment of the ATI digitizer of FIG.1 including a calibration system according to embodiments of theinvention.

FIG. 16 is a block diagram of another embodiment of the ATI digitizer ofFIG. 1 including a calibration system according to embodiments of theinvention.

FIG. 17 is a two-dimensional array illustrating how filter coefficientsmay be stored and indexed during calibration of an ATI digitizeraccording to embodiments of the invention.

DETAILED DESCRIPTION

This disclosure describes embodiments of an ADC system for a test andmeasurement instrument using harmonic mixing.

FIG. 1 is a block diagram of an ADC system for a test and measurementinstrument using harmonic mixing according to an embodiment of theinvention. In this embodiment, the instrument includes a splitter 10configured to split an input signal 12 having a particular frequencyspectrum into multiple split signals 14 and 16, each split signalincluding substantially the entire spectrum of the input signal 12. Asplitter 10 can be any variety of circuitry that can split the inputsignal 12 into multiple signals. For example, the splitter 10 can be aresistive divider. Thus, substantially all frequency components of theinput signal 12 can be present in each split signal 14 and 16. However,depending on the number of paths, harmonic signals used, or the like,the frequency responses for various split signals of a splitter 10 canbe different.

The split signals 14 and 16 are inputs to harmonic mixers 18 and 24,respectively. Harmonic mixer 18 is configured to mix the split signal 14with a harmonic signal 20 to generate a mixed signal 22. Similarly,harmonic mixer 24 is configured to mix the split signal 16 with aharmonic signal 26 to generate a mixed signal 28.

As used herein, a harmonic mixer is a device configured to mix a signalwith multiple harmonics. Although multiplication and/or mixing has beendescribed in connection with harmonic mixing, as will be described infurther detail below, a device that has the effect of multiplying asignal with multiple harmonics can be used as a harmonic mixer.

In some embodiments, the multiple harmonics can include a zero-orderharmonic, or a DC component. For example, in some embodiments, theharmonic signal 20 can be a signal represented by equation (1):1+2 cos(2πF₁t)  (1)

Here F₁ represents the first-order harmonic and t represents time. Thus,a signal having the form of equation (1) has harmonics at DC and atfrequency F₁.

Harmonic signal 26 can be a signal represented by equation (2)1−2 cos(2πF₁t)  (2)

Similar to harmonic signal 20, harmonic signal 26 has harmonics at DCand frequency F₁. However, the first-order harmonic at frequency F₁ isout of phase by 180 degrees relative to the similar first-order harmonicin harmonic signal 20.

A digitizer 30 is configured to digitize mixed signal 22. Similarly, adigitizer 32 is configured to digitize mixed signal 28. The digitizers30 and 32 can be any variety of digitizer. Although not illustrated,each digitizer 30 and 32 can have a preamplifier, filter, attenuator,and other analog circuitry as needed. Thus, the mixed signal 22 input tothe digitizer 30, for example, can be amplified, attenuated, orotherwise filtered before digitization.

The digitizers 30 and 32 are configured to operate at an effectivesample rate. In some embodiments, the digitizer 30 can include a singleanalog to digital converter (ADC). However, in other embodiments, thedigitizer 30 can include multiple interleaved ADCs operating at lowersample rates to achieve a higher effective sample rate.

A first-order harmonic of at least one of the harmonic signals 20 and 26is different from an effective sample rate of at least one of thedigitizers 30 and 32. For example, the first-order harmonic F₁ of theharmonic signal 20 could be 34 GHz. A sample rate of the digitizer 30could be 50 GS/s. Thus, the first-order harmonic F₁ is different fromthe effective sample rate.

In some embodiments, the first-order harmonic of a harmonic signal neednot be an integer multiple or sub-multiple of the effective sample rateof the at least one of the digitizers. In other words, in someembodiments, the first-order harmonic of a harmonic signal associatedwith the harmonic mixers is not an integer multiple or sub-multiple ofthe effective sample rate of the at least one of the digitizers.

In some embodiments, the first-order harmonic of a harmonic signal canbe between the effective sample rate of the at least one of thedigitizers and one half of the effective sample rate of the at least oneof the digitizers. In particular, as will be described in further detailbelow, such a frequency allows higher frequency components above and/orbelow the first-order harmonic to be mixed down in frequency to be belowone half of the sample rate of the digitizer 30. Thus, such frequencycomponents can be digitized effectively by the digitizer 30.

It should be understood that all bands of the input signal 12 go throughall paths. In other words, when more than one channel is combined forprocessing a single input signal 12, each channel or path receivessubstantially the entire bandwidth of the input signal 12. As the inputsignal 12 is transmitted through all of the digitizers, the signal tonoise ratio is significantly improved.

A filter 36 can be configured to filter the digitized mixed signal 34from digitizer 30. Similarly, a filter 42 can be configured to filterthe mixed signal 40 from digitizer 32. Harmonic mixers 46 and 52 areconfigured to mix the filtered mixed signals 38 and 44 with harmonicsignals 48 and 54, respectively. In some embodiments, the harmonicsignals 48 and 54 can be substantially similar in frequency and phase tothe corresponding harmonic signals 20 and 26. While the harmonic signals20 and 26 are analog signals, and the harmonic signals 48 and 54 aredigital signals, the scaling factors for these harmonic signals can bethe same or similar to each other. The output signals 50 and 56 arereferred to as remixed signals 50 and 56. A combiner 58 is configured tocombine the remixed signals 50 and 56 into a reconstructed input signal60. In some embodiments, the combiner 58 can implement more than mereaddition of signals. For example, averaging, filtering, scaling, or thelike can be implemented in the combiner 58.

The filters 36 and 42, the harmonic mixers 46 and 52, harmonic signals48 and 54, the combiner 58, and other associated elements can beimplemented digitally. For example, a digital signal processor (DSP),microprocessor, programmable logic device, general purpose processor, orother processing system with appropriate peripheral devices as desiredcan be used to implement the functionality of the processing of thedigitized signals. Any variation between complete integration to fullydiscrete components can be used to implement the functionality.

Some form of synchronization of the harmonic signals 20, 26, 48, and 54is used. For example, the harmonics of the harmonic signals 20 and 26can be locked to a clock related to the digitizers 30 and 32. In anotherexample, the harmonic signal can be digitized. Thus, the first-orderharmonic would be available to synchronize the harmonic signals 48 and54. In another example, out-of-band tones can be added to one or more ofthe mixed signals 22 and 28. Using a first-order harmonic of 34 GHz,19.125 GHz and 21.25 GHz tones, or 9/16 and 10/16 of 34 GHz, can beadded to the mixed signal 22. Since these tones are outside of abandwidth of the filtering eventually established by filter 36, i.e.,approximately 18 GHz depending on the transition band, the tones canhave a substantially negligible effect on the reconstructed signal 60.However, as the tones can be less than a Nyquist frequency, i.e. lessthan 25 GHz for a 50 GS/s sample rate, the tones can be acquired byusing the digitized mixed signal 34 before filtering. Regardless of thetechnique used, a phase and frequency relationship between the harmonicsignals 20 and 26 and the digital harmonic signals 48 and 54 can bemaintained.

FIGS. 2-8 illustrate examples of spectral components of various signalsin the ADC system for the test and measurement instrument of FIG. 1.Referring to FIGS. 1 and 2, spectrum 100 can be a spectrum of the inputsignal 12 and hence, the split signal 14. Using the above example of theharmonic signal defined in equation (1), a DC component of the splitsignal 14 is passed, as represented by spectrum 100. However, thespectrum 100 in the input signal 12 is also mixed with the first-orderharmonic at frequency F₁. The resulting spectrum 102 is the product ofsuch mixing. Thus, the mixed signal 22 includes components of spectrum100 and spectrum 102. Here, and in other figures, the spectralcomponents are illustrated as separate and overlapping however, theactual spectrum would be the combination of the spectra 100 and 102.

Referring to FIGS. 1 and 3, spectrum 110 similarly represents componentsof the mixed signal 28 due to the mixing of input signal 12 with the DCharmonic of the harmonic signal 26. However, in contrast to FIG. 2, thespectrum 112 has a 180 degree phase difference relative to the spectrum102 of FIG. 2. As described above, the first-order harmonic of theharmonic signal 26 is phase shifted by 180 degrees from the first-orderharmonic of the harmonic signal 20. This 180 degree phase shift in theharmonic signal 26 induces a 180 degree phase shift in the spectrum 112.The 180 degree phase difference is illustrated as a dashed line.

FIGS. 4 and 5 represent the spectrums of the filtered mixed signals 38and 44. In some embodiments, the filtering can be a function of inherentfiltering of the corresponding digitizers 30 and 32, the filters 36 and42, or the like. Although filtering is illustrated in FIG. 1 asoccurring after the digitizers 36 and 42, filtering can be performed inother locations. For example, some filtering can occur prior todigitization. The mixed signals 22 and 28 could be filtered with a lowpass filter having a cutoff frequency near one half of the effectivesample rate of the digitizers 30 and 32. The filtering of filters 36 and42 can add to such inherent and/or induced filtering.

In some embodiments, the net filtering of the mixed signals 22 and 28can result in a frequency response that is substantially complementaryabout one half of a frequency of the first-order harmonic of theharmonic signals 20 and 26. That is, the frequency response at a givenoffset higher than frequency F₁/2 and the frequency response at a givenoffset lower than frequency F₁/2 can add to one. Although one has beenused as an example, other values can be used as desired, such as forscaling of signals. Furthermore, the above example is described as anideal case. That is, the implemented filtering can have differentresponse to account for non-ideal components, calibration, or the like.

In a particular example of the frequency response, using the 34 GHz F₁described above, frequency F₁/2 can be 17 GHz. From DC to 16 GHz thefrequency response can be one. From 16 to 18 GHz, the frequency responsecan linearly change from one to zero, passing through ½ at 17 GHz.

The resulting spectral components in FIG. 4, representing the filteredmixed signal 38 include a lower frequency portion of spectrum 100,illustrated by spectrum 120, and a lower frequency portion of spectrum102, illustrated by spectrum 122. Note that due to the mixing, spectrum122 includes frequency components of a higher sub-band of spectrum 100,albeit reversed in frequency. Similarly, the spectral components 130 and132 of FIG. 5 correspond to the lower frequency components of spectra110 and 112 of FIG. 3. The 180 degree phase relationship of spectrum 112is preserved in spectrum 132.

Accordingly, through the harmonic mixing, two sub-bands of an inputsignal 12 have been digitized even though the span of the sub-bandswould have exceeded a Nyquist bandwidth associated with the digitizers30 and 32. In this embodiment, each mixed signal, whether analog,digital, filtered, or the like, includes components of each sub-band ofthe input signal 12. That is, in this example, each signal from themixed signals 22 and 28 to the filtered digitized mixed signal 38 and 44includes both a low frequency sub-band and a high frequency sub-band ofspectrum 100.

In particular, the sub-bands of the input signal 12 have been frequencyshifted to be within the bandwidth of a baseband sub-band. In someembodiments, each sub-band of the input signal 12 can be frequencyshifted to be within the bandwidth of the single sub-band. However,depending on the number of sub-bands, and the harmonic signals, eachsub-band may not be present in each mixed signal.

FIGS. 6 and 7 represent the spectra of the remixed signals 50 and 56.Referring to FIGS. 1 and 6, the spectrum represents the remixed signal50. As described above the filtered digitized mixed signal 38 can bemixed in the harmonic mixer 46 with the harmonic signal 48 that issubstantially similar in frequency and phase to the harmonic signal 20.Accordingly, the spectra of FIG. 4 are mixed with a DC component and afirst-order harmonic.

Spectra 140 and 142 represent the spectra from mixing the spectra 120and 122 of FIG. 4 with the DC component. Spectrum 144 represents theresult of mixing the spectrum 120 with the first-order harmonic. Spectra146 and 148 represent the mixing of spectrum 122 of FIG. 4 with thefirst-order harmonic.

Similarly, FIG. 7 represents the spectra of the remixed signal 56.Spectra 150 and 152 represent the mixing of the DC component with thespectra of FIG. 5. Spectrum 154 represents the mixing of the first-orderharmonic of the harmonic signal 54 with the spectrum 130 of FIG. 5. Inparticular, as the first-order harmonic of harmonic signal 54 has arelative 180 degree phase shift, the resulting spectrum 154 also has a180 degree phase shift, represented by the dashed line.

Spectrum 132 of FIG. 5 is also mixed with the first-order harmonic ofharmonic signal 54; however, the spectrum 132 already had a 180 degreeinduced phase shift. Thus, the additional 180 degree phase shift resultsin an effective 0 degree phase shift, represented by the solid line ofspectra 156 and 158.

FIG. 8 illustrates a spectrum 160 of the reconstructed input signal 60of FIG. 1. Spectra 162 and 164 represent the component sub-bands formingthe spectrum 160. Spectrum 166 represents an additional sideband fromthe mixing described with respect to FIGS. 6 and 7. In this embodiment,spectrum 166 can be filtered out; however, in other embodimentssub-bands can extend beyond the first-order harmonic frequency F₁. Insuch an embodiment, spectrum 166, being generated from a lower frequencysub-band, can be eliminated through destructive combination.

Due to the relative phasing of the components of the remixed signals 50and 56, sub-bands in their original frequency range combineconstructively, while sub-bands outside of their original frequencyrange are phased to combine destructively. Referring to FIGS. 6-8, whencombined, spectra 140 and 150 combine constructively, resulting inspectrum 162. Spectra 142 and 152 combine destructively as the spectraare out of phase by 180 degrees. Thus, of the spectra within thebaseband sub-band, the remaining sub-band is the original sub-band.

Similarly, for the sub-band from approximately F₁/2 to F₁, spectra 146and 156 combine constructively into spectrum 164, while spectra 144 and154 combine destructively. Spectra 148 and 158 combine constructivelyinto spectrum 166; however, spectrum 166 can be filtered out as it isbeyond the expected input frequency range which in this case is aboutless than frequency F₁.

As illustrated by spectra 162 and 164, a transition occurs aroundfrequency F₁/2. This transition is the result of the filtering describedabove in reference to FIGS. 4 and 5. In particular, the slopes ofspectrum 162 and spectrum 164 are complementary. Thus, when thefrequency components of the spectrums 162 and 164 are combined, theresulting portion of the spectrum 160 substantially matches the originalfrequency spectrum.

Accordingly, by mixing the input signal 12 with various harmonicsignals, sub-bands of the input signal 12 can be passed through thelower bandwidth of a digitizer. Although the mixed signals includedoverlapping sub-bands, because of the phasing of the harmonic signals,the sub-bands combine constructively and destructively when combined asdescribed above to create a substantially accurate representation of theinput signal 12.

FIGS. 9-12 are block diagrams of examples of harmonic mixers of FIG. 1.In some embodiments, a mixer can be used to mix the split signals 14 and16 with the respective harmonic signals 20 and 26. A mixer that can passDC and baseband signals on all ports can be used as a harmonic mixer.

FIGS. 9A and 9B illustrate examples of a harmonic mixer, which canrepresent any one or more of the harmonic mixers 18, 24, 46, and/or 52discussed above. FIG. 9A illustrates a 2-way time-interleaving switch.FIG. 9B illustrates an N-way time-interleaving switch.

In these embodiments, switches 180 and/or 181 are configured to receivean input signal 182. When using the 2-way switch 180, the input signal182 is switched to outputs 184 and 186 in response to a control signal188. When using the N-way switch 181, the input signal 182 is switchedto the outputs 184, 186, on through to the Nth output 187, in responseto the control signal 188. For example, the switch 181 can be athree-throw switch, a four-throw switch, etc., up to an N-throw switch,which causes the input signal 182 to spend 1/Nth of its time at eachpoint or output. As further paths and sub-bands are added, the harmonicsof the harmonic signals can be appropriately phased. In someembodiments, the relative phase shifts of the harmonic signals can bespaced in phase by time shifts of one period divided by the number ofsub-bands.

As the pulses get shorter compared to the overall clock cycle, theharmonic content gets richer. For instance, for a two-way or a three-wayswitch, the zero-order harmonic (DC) and the first-order harmonic areused. For a four-way or five-way switch, the zero-order harmonic, thefirst-order harmonic, and a second-order harmonic can be used. For asix-way or seven-way switch, the zero-order harmonic, the first-orderharmonic, a second-order harmonic, and a third-order harmonic can beused. As N increases, the pulses get narrower, thereby generating thericher harmonic content. The control signal 188 can be a signal having afundamental frequency of the first-order harmonic, or other suitableharmonic frequency, described above.

All bands of the input signal 182 go through all paths, i.e., to each ofthe outputs paths (e.g., 184, 186, through the Nth output 187).

For example, referring to switch 180, the control signal 188 can be asquare wave with a fundamental frequency of 34 GHz. As a result of theswitching, output 184 will receive the input signal 182 during onehalf-cycle of the control signal and will be approximately zero duringthe opposite half-cycle. In effect, the output 184 is the input signal182 multiplied by a square wave oscillating between zero and one at 34GHz. Such a square wave can be represented by equation (3).

$\begin{matrix}{0.5 + {\frac{2}{\pi}{\sin\left( {2\;\pi\; F_{1}t} \right)}} + {\frac{2}{3\;\pi}{\sin\left( {6\;\pi\; F_{1}t} \right)}} + \ldots} & (3)\end{matrix}$

Equation (3) is the Taylor series expansion of such a square wave. TheDC and first two harmonics are listed. Here F₁ is 34 GHz. Although themagnitudes of the components are different, equations (1) and (3)include similar harmonics.

Output 186 is similar to output 184; however, the time period over whichthe input signal 182 is routed to the output 186 is inverted relative tooutput 184. The effect is again similar to multiplying the input signal182 with a square wave defined by equation (4).

$\begin{matrix}{0.5 - {\frac{2}{\pi}{\sin\left( {2\;\pi\; F_{1}t} \right)}} - {\frac{2}{3\;\pi}{\sin\left( {6\;\pi\; F_{1}t} \right)}} + \ldots} & (4)\end{matrix}$

Similar to equation (3), equation (4) is similar to the harmonic signaldescribed in equation (2) above. Thus, the multiplication effect of theswitching of the switch 180 is substantially similar to the mixing of asplit signal with the harmonic signal described above. In addition, inthis example, the switch can act as both the splitter 10 and harmonicmixers 18 and 24. However, in other embodiments, the switch 180 could bea single pole single throw switch and act as a single harmonic mixer.

Although the relative magnitudes of the DC component and the first-orderharmonic are different, such imbalance can be corrected through acompensation filter in the appropriate path. For example, the sub-banddescribed above between frequency F₁/2 and frequency F₁ can have adifferent gain applied during recombination in the combiner 58 than abaseband sub-band.

In addition, equations (3) and (4) above also list third-orderharmonics. In some embodiments, the third-order harmonics may bedesired. However, if not, the effect of such harmonics can becompensated with appropriate filtering. For example, the input signal 12can be filtered to remove frequency components above frequency F₁. Thus,such frequency components would not be present to mix with a frequencyat 3*F₁. Moreover, filtering before a digitizer can remove any higherorder frequency components that may otherwise affect the digitizedsignal due to aliasing.

In the event of interleaving errors due to analog mismatch, hardwareadjustments can be made for mixing clock amplitude and phase. Theadjustments can then be calibrated to minimize interleave mismatchspurs. Alternatively, or in addition to the above approach, hardwaremismatches can be characterized, and a linear, time-varying correctionfilter can be used to cancel the interleave spurs. Further, in somecases, the switches might not always operate perfectly. For example, anerrant switch might spend more time in one direction than the other,thereby causing a skewed duty cycle. The digital harmonic mixers 46 and52 can be configured to compensate for phase or amplitude errors thatmay be present in the analog harmonic signals 20 and/or 26 by makingsubtle adjustments to the amplitude or phase of the digital harmonicsignals 48 and/or 54.

FIG. 10 is an example of another harmonic mixer. A switching circuit 200is configured to switch two input signals 202 and 204 alternatively tooutputs 208 and 210 in response to the control signal 206. The controlsignal 206 can again be a square wave or other similar signal to enablethe switches of the switching circuit 200 to switch. During onehalf-cycle of the control signal 206, input signal 202 is switched tooutput 208 while input signal 204 is switched to output 210. During theother half-cycle, the input signal 202 is switched to output 210 whileinput signal 204 is switched to output 208.

In some embodiments, the input signal 204 can be an inverted and scaledversion of the input signal 202. The result of such inputs and theswitching described above is a rebalancing of the DC and other harmonicsfrom the levels described above with respect to the switch 180 of FIG.9A. For example, input signal 204 can be a fractional inverted versionof the inputs signal 202. Instead of switching between 1 and 0 with theswitch 180 of FIG. 9A, the effective output of outputs 208 and 210 canbe switching between 1 and (2−π)/(2+π), for example. Thus, the amplitudeand DC level can be adjusted as desired to create the desired balancebetween the harmonics.

FIG. 11 illustrates an alternative example of a harmonic mixer. Theharmonic mixer 170 includes a splitter 172, a mixer 175, and a combiner177. The splitter 172 is configured to split an input signal 171 intosignals 173 and 174. Signal 174 is input to the combiner 177. As signal174 is not mixed with another signal, signal 174 can act as the DCcomponent of a harmonic mixer described above.

Signal 173 is input to the mixer 175. A signal 176 is mixed with thesignal 173. In some embodiments, signal 176 can be a single harmonic,such as the frequency F₁ described above. If additional harmonics aredesired, additional mixers can be provided and the respective outputscombined in combiner 177.

In another embodiment, the signal 176 can include multiple harmonics. Aslong as the bandwidth of the ports of the mixer 175 can accommodate thedesired frequency ranges, a single mixer 175 can be used. However, sincethe DC component of the harmonic signals described above is passed tothe combiner 177 by a different path, the ports of the mixer receivingsignals 173 and 176 need not operate to DC. Accordingly, a wider varietyof mixers may be used. Once the signals 179 and 174 are combined in thecombiner 177, the output signal 178 can be substantially similar to amixed signal described above.

In some embodiments, the splitter 172 can, but need not split the inputsignal 171 symmetrically. For example, a side of the splitter thatoutputs signal 174 may have a bandwidth that is at or above thefiltering cutoff frequency described above. A side of the splitter 172that outputs signal 173 can have a frequency range centered on aharmonic of the signal 176 and a bandwidth of twice or greater of thefiltering cutoff frequency described above. In other words, thefrequency response of the splitter 172 need not be equal for each pathand can be tailored as desired.

FIG. 12 is another example of a harmonic mixer of the general topologyof FIG. 9A. In this embodiment, a harmonic signal 224 can be input to adiode ring 220 similar to a mixer through transformer 225. The inputsignal 222 can be input to a tap of the transformer 225. Accordingly,depending on the harmonic signal 224, the input signal 222 can beswitched between outputs 226 and 228. For example, the harmonic signal224 causes either the left diodes 227 to turn on when the bottom of thetransformer is positive and the top is negative, or the right diodes 229to turn on when the polarity of the transformer is reversed. In thismanner, the input signal 222 is alternately routed to the output 228 andthe output 226. In some embodiments, an additional diode ring could beused to terminate the outputs and/or inject an inverted portion of asub-band of the input signal 222 to achieve a higher gain, compensatefor imbalanced harmonics, or the like, as in the topology of FIG. 10.

In some embodiments, two paths and two overlapping sub-bands areimplemented. However, as mentioned above, any number of paths andsub-bands can be used. In such embodiments, the number of harmonics usedcan be equal to one plus one half of a number of sub-bands, roundeddown, where DC is included as a zero-order harmonic. For example, forthree sub-bands, only two harmonics can be used. Using the abovefrequency ranges as an example, the first-order harmonic can frequencyshift frequencies higher than frequency F₁ to the baseband sub-band. Thefirst-order harmonics of the harmonic signals can be phased with 120degree relative phase shifts.

Accordingly, when a sub-band is in the proper frequency range duringcombination in the combiner 58, the sub-band spectra will have the samephase shift, such as a 0 degree relative phase shift. In contrast, thethree components of a sub-band in the incorrect frequency range wouldoffset in phase from one another by 120 degrees. The resulting spectrawould destructively combine to eliminate the incorrect sub-band. Asfurther paths and sub-bands are added, the harmonics of the harmonicsignals can be appropriately phased. In some embodiments, the relativephase shifts of the harmonic signals can be spaced in phase by timeshifts of one period divided by the number of sub-bands.

Although embodiments have been described above where digitized signalscan be substantially immediately processed, such processing afterdigitization can be deferred as desired. For example, the digitized datafrom digitizers 30 and 32 can be stored in a memory for subsequentprocessing.

Moreover, although the digital filtering, mixing, and combining havebeen described as discrete operations, such operations can be combined,incorporated into other functions, or the like. In addition, as theabove discussion assumed ideal components, additional compensation, canbe introduced into such processing as appropriate to correct fornon-ideal components. Furthermore, when processing the digitizedsignals, changing frequency ranges, mixing, and the like can result in ahigher sample rate to represent such changes. The digitized signals canbe upsampled, interpolated, or the like as appropriate.

As mentioned above, the digital harmonic mixers 46 and 52 can beconfigured to compensate for phase errors that may be present in theanalog harmonic signals 20 and/or 26 by making subtle adjustments to theamplitude or phase of the digital harmonic signals 48 and/or 54. Shiftsin delays of various components over time or temperature may causeunacceptable amounts of phase shift. Delay shifts in the circuitrygenerating the analog harmonic signals, in the analog mixers, and/or inthe analog-to-digital channel aperture would all contribute to a phaseerror between the analog mixers 18 and 24 and the digital mixers 46 and52, respectively.

If the phase error is uncorrected, the mixing phase error will effect anequal phase error in the frequency components within the upper bands ofthe reconstructed waveform, leading to distortion in the step responseof the system. Additionally, amplitude errors will result for frequencycomponents within the cross-over band, as the unconverted and thetwice-converted vectors representing the frequency component, as will bediscussed in more detail below, will not be properly aligned when addedtogether near the end of the reconstruction process.

Some embodiments of the test and measurement instrument contain acompensation oscillator 300 and a switch 302 as shown in FIG. 13. Acompensation oscillation signal 304 from the compensation oscillator 300can be switched into the input of an ATI digitizer, described above, viaswitch 302. The compensation oscillator 300 can be used to determine thephase and amplitude errors, as discussed in more detail below, so thephase and amplitude errors can be removed.

The compensation oscillator 300 and switch 302 are included within anintegrated circuit for the ATI digitizer so the compensation oscillator300 adds little cost or power overhead to the system. Further, thecompensation oscillator 300 is tunable over a frequency range wider thanthe integrated circuit process uncertainty of the center frequency,ensuring that the system can find an appropriate tune voltage to placethe compensation oscillator 300 frequency within the cross-over band.

Since the frequency of the compensation signal 304 from the compensationoscillator 300 is tuned to be within the cross-over band, thecompensation signal 304 travels through an ADC channel of the ATIdigitizer both at its original frequency and as a down-converted andsubsequently digitally-up-converted frequency component. The phase ofthe original frequency component of the compensation signal 304 is notimpacted by the phase error between the analog and digital harmonicmixing signals, but the phase of the twice-converted component isimpacted.

A phase error value can be determined based on comparing the originalfrequency component of the compensation signal 304 that was not affectedby the phase error and the twice-converted component which has beenaffected by the phase error traveling through one ADC channel of the ATIdigitizer. Comparing these values provides the phase error value betweenthe analog and digital mixers in that ADC channel. The phase error canthen be used to adjust the mixing function of either the analog mixer 18or the digital mixer 46, if in the upper ADC channel. Adjusting themixing function of one of the mixers 18 or 46, or if in the lower ADCchannel in FIG. 13, mixers 24 and 52, allows for the phase error to beremoved from the reconstructed waveforms. Alternatively, the phase errormay be removed by changing the delay of digital filter 36 in the upperADC channel or digital filter 42 in the lower ADC channel, as a phaseshift in either input to the digital mixers 46 and 52 will effect aphase shift in the output.

Preferably, the compensation oscillator 300 compensation signal 304 isswitched into the input via switch 302 immediately after an acquisitionof a signal to be tested, rather than beforehand, as the measurement ofthe phase error can be applied to correct the mixing functions of thedigital mixers 46 and 52 or the delays of digital filters 36 and 42. Theinformation is not needed until the ATI reconstruction of the signaloccurs post-acquisition.

As seen in FIG. 14, a memory 400 may be provided between digitizer 30and filter 36 in the upper ADC channel and a memory 402 betweendigitizer 32 and filter 42 in the lower ADC channel. An acquisition canbe performed and the digitized mixed signal 34 or the digitized mixedsignal 40 can be stored in memories 400 and 402, respectively, beforebeing sent to filters 36 and 42, respectively.

After the digitized mixed signals 34 and 40 have been stored in memories400 and 402, respectively, switch 302 can be triggered to automaticallyprovide the compensation signal 304 from the compensation oscillator 300without a user input. For example, a digital signal processor (DSP),microprocessor, programmable logic device, general purpose processor, orother processing system with appropriate peripheral devices as desiredcan be used to automatically switch to the compensation signal 304 fromthe compensation oscillator 300. The phase error can be determined asdiscussed above, and the mixing functions of the digital mixers 46 and52 or the delays of digital filters 36 and 42 can be adjusted. Once themixing functions or filter delays have been modified based on the phaseerror, then the digitized mixed signals 34 and 40 can be processedthrough the remaining portions of the ADC channels as discussed abovewith respect to FIG. 1.

Running the compensation after the acquisition minimizes the opportunityfor a phase drift between the compensation and acquisition modes. Acompensation run before a signal acquisition may be performed anarbitrary time before a signal acquisition, as there is no way ofknowing how long an acquisition will be running waiting for a triggerevent. However, if the system phase stability is sufficiently good, thenthe compensation process can be run before acquisition. Further, if auser decides a compensation is desired, the user may begin thecompensation via a menu on the test and measurement instrument.

When the compensation oscillator 300 is enabled, the input signalacquisition is automatically switched off via switch 302 and replacedwith the compensation signal 304, allowing the compensation to runwithout requiring user interaction. Further, the compensation signal 304may be switched on after a trigger event has been detected, without userinput, using a processor, or the like, as discussed above. Thecompensation oscillator 300 can also automatically be switched on afterevery signal acquisition to provide the compensation signal 304 todetermine a phase or amplitude error.

Digitizers 30 and 32 may suffer from phase drift between theirrespective sampling clocks, such that the unconverted signals passingthrough the analog mixers are not sampled at the same time. Also,digitizers 30 and 32 may themselves employ interleaving techniques, suchas synchronous time interleaving, to achieve their effective samplerates. In that case, the interleaved acquisition pipes within digitizers30 and 32 may similarly suffer from phase drift of their respectivesample clocks. Compensation oscillator 300 can also be used to provide acompensation signal 304 through the ATI front-end to each ADC channelfor the purpose of determining phase errors of acquisition pipes withinand/or between the ADC channels. This can be accomplished by tuning thecompensation oscillator 300 out of the cross-over band so that only onetone is output from the analog mixers 18 and 24 within the bandwidth ofeach ADC channel. Alternatively, if compensation oscillator 300frequency is left within the cross-over band, a sine-fit algorithm usedto measure the phases of each ADC pipe could be set to fit just theunconverted frequency component and not an image component, or viceversa.

The measured phase errors may be used to adjust the phase response ofdigital filters 36 and 42 to correct for the impact of the sampling timeerrors. Adjusting the delay of one digital filter with respect to theother digital filter may compensate for phase error between thedigitizers 30 and 32. If the digitizers 30 and 32 are internallyinterleaved, pipe-dependent phase shifts may be applied within eachdigital filter 36 and 42 to compensate phase errors within eachdigitizer 30 and 32, respectively. Alternatively, the phase errors couldbe used to adjust the sample-clock timing of the acquisition pipes tominimize the error in subsequent acquisitions.

The compensation oscillator 300 can be built from a cross-coupled NPNdifferential pair amplifier, to generate negative resistance, and ashorted transmission-line stub, to set a nominal frequency. Thecompensation oscillator 300 is turned on and tuned by setting an emittercurrent in the differential pair amplifier. Once the current is highenough to provide sufficient transconductance to support oscillation,further increase in current serves to increase the devices' inputcapacitance, which in turn loads the transmission-lines and lowers theresonant frequency. That is, the tunable compensation oscillator 300 istuned predominantly through varying an input capacitance of at least onebipolar junction transistor.

Use of the input capacitance tuning provides a relatively large andlinear tune range compared to varactor tuning at these frequencies. Thelarge tune range is helpful to overcome process modeling uncertainty andprocess variability. If the large tune range of the compensationoscillator 300 causes excessive frequency instability within theduration of the compensation acquisition, the acquired compensationrecord can be split into multiple shorter segments and analyzed forphase errors using separate sine-fits with potentially differentfrequencies in each of the segments. The measured phase error betweenthe unconverted and twice-converted components in each segmentrepresents the phase error between the analog and digital harmonicsignals, and is independent of the exact frequency of the compensationsignal used. Thus the results of the segment phase error measurementsmay be averaged to gain the same noise immunity as the single longrecord.

As mentioned briefly above, an amplitude error can also be determinedusing the compensation oscillator 300. To determine the amplitude error,the input of the harmonic mixer, also referred to as an ATI digitizer,such as the digitizer illustrated in FIG. 13, may be swept with thecompensation signal 304 over at least two frequencies symmetricallyopposed within the cross-over band. When the input frequency is belowthe center of the cross-over band, the ratio of amplitudes of thedigitized signal at the converted frequency and the input frequency willbe the product of the conversion gain and the digitizer frequencyresponse roll-off. When the input frequency is symmetrically above thecenter of the cross-over band, the ratio of amplitudes of the digitizedsignal at the converted frequency and the input frequency will be theratio of the conversion gain and the digitizer frequency roll-off. Thegeometric mean of these two amplitude ratios then represents theconversion gain. The amplitude of the analog mixing functions 20, 26 orthe digital mixing functions 48, 54 may then be adjusted to bring theconversion gain to the desired value, generally 1.0.

With reference to FIGS. 14 and 15, embodiments of the invention alsoinclude systems for performing real-time calibration of digitizingsystems in situations where an internal compensation oscillator, such asthe oscillator 300 in FIGS. 13 and 14 may not be capable of generatingsignals that span the entire bandwidth of the input signal 12. Forexample, for a test and measurement system that accepts input signalshaving frequencies up to 70 GHz, the oscillator 300 may produce signalsin the range of, for example, 30 to 40 GHz.

Further embodiments provide a system and methods to compensate forhardware errors, such as those that vary as a function of inputfrequency that therefore are best characterized over the full inputrange. As described below, embodiments of the invention allow forfull-range characterization during a factory calibration, and furtheruse the compensation oscillator to measure a subset of hardware errors,preferably delay and skew, and use the measured hardware errors as anindex into a factory look-up table that stores pre-determined sets ofcompensating filter coefficients.

As described above, hardware errors can be characterized, and a linear,time-varying correction filter can be used to cancel the interleavespurs. For example, a linear, time-varying filter, such as a linear,time-periodic or “LTP” filter 70 may be inserted at the end of thereconstruction DSP chain, such as at the output of the combiner 58, tocorrect hardware mismatch errors. Such hardware errors may include thosebetween the interleaved ADC sub-channels within each ADC channel, andbetween the two ADC channels themselves. An LTP correction filter canalso correct more complicated hardware errors than the simple timingerrors described above, for example time errors that vary as a functionof the input frequency. Although other filter parameters are possible,an example LTP filter 70 has an 80 ps period, matching the sampleinterval of the interleaved ADC sub-channels and being an integermultiple of the 13¼ ps period of the mixing clock. In some embodiments,a 75 GHz mixing clock, which is coupled to the mixers, such as mixers 18and 24, is generated from a 12.5 GHz system clock. Due to hardwareimperfections in the frequency multiplier, the generated mixing clockcontains some residual spurs at integer multiples of 12.5 GHz besidesthe desired 6× multiple.

Such mixing spurs create spurs in the digitized record at exactly thesame frequencies as spurs produced by ADC sub-channel interleavemismatch, and thus, using techniques described herein, may also becorrected by the same LTP filter topology. However, calibrating the LTPfilter to correct for frequency-dependent timing errors and/orclock-spur-related mixing errors requires measuring the hardware errorsat multiple input frequencies spanning the full input bandwidth of thedigitizer system. It is generally cost-prohibitive to build suchfull-range signal generation capability into the digitizer systemitself, so the calibration of the LTP filter must be performed in thefactory with access to a suitable signal source.

Although oscilloscopes such as described here are calibrated at thefactory, conditions in which they operate may not match those of thefactory. The testing room may change temperature and/or humidity, forexample, or the performance of some components may drift over time. Ingeneral, the largest hardware drifts for test and measurement systemscaused by environmental conditions (temperature and humidity) exhibitthemselves in the relative clock phasing of the ADC channels withrespect to each other, and with respect to the analog mixing function.To the extent these phases drifted just with temperature, it would berelatively easy to characterize their temperature coefficients andadjust the hardware in real time as a function of a measuredtemperature. However, it is relatively difficult to accurately measurethe amount of humidity absorbed into the printed circuit boards on whichthe oscilloscope components are formed. Also, some errors caused byenvironmental conditions could be corrected by adjusting the phases ofthe digital mixing functions and/or the delays of the digital filtersbefore the digital mixers. Such calibration techniques, however, may notfully correct the change in phase of the signal spurs caused by theunwanted 12.5 GHz harmonics in the 75 GHz analog mixing function withrespect to the phase of the ADC sub-channel interleave mismatch spurswhich occur at the same frequencies. Thus, as described in more detailbelow, embodiments of the invention directly monitor the clock phasesand skew of clocks driving mixers in the oscilloscope using the built-incalibration oscillator. Then the DSP is modified based on the measuredclock phase and skew to account for the measured errors after-the-fact.

In some embodiments, the modification of the DSP, such as the DSPillustrated in FIGS. 15 and 16 is performed by generating a new set ofcoefficients for the LTP filter 70. In other embodiments, themodification of the DSP may be performed by adjusting clocks driving themixers 46, 52, such as by using the time adjustors 74, 76 in FIGS. 15and 16, and also generating a new set of coefficients for the LTP filter70. Using either method allows correction for shifts in both clock delayand skew. In one embodiment, clock delay is measured as a delay betweenthe clocks for mixers 18, 24 in the analog domain as compared to theclocks for digitizers 30, 32 in the digital domain. Also, clock skew maybe measured by measuring the particular skew between the clocks fordigitizers 30, 32 themselves.

For a particular hardware state (temperature, humidity, etc.), an idealset of coefficients for the LTP correction filter 70 may be calculatedby a method such as described in U.S. Pat. No. 8,698,659, which isincorporated by reference herein. However, as set forth in thatreference, performing such calculations requires access to a signalsource that spans the full frequency range of the digitizer. Thefrequency range of the compensation oscillator 300, such as illustratedin FIGS. 13 and 14, are generally not tunable over the entire range ofthe digitizer, and instead is typically tunable over only a portion ofthe full frequency range. Thus calculating filter coefficients accordingto techniques described in the '659 patent cannot be used in all cases.Instead, for cases where the compensation oscillator may not include thefull range of the digitizer, techniques according to embodiments of theinvention may be used.

Embodiments of the invention include a Look Up Table (LUT) 72 coupled tothe LTP filter 70, as illustrated in FIGS. 15 and 16. The LUT 72 storesfilter coefficients for the LTP filter 70 to correct for various runtime conditions. Then, in operation, the run time conditions arecommunicated to the LUT 72, which selects the appropriate filtercoefficients for the LTP filter 70 to correct for the present run timecondition.

To initially generate the filter coefficients stored in the LUT 72, theinstrument is first factory calibrated using a full-range oscillator.Then, an error condition is artificially introduced into the instrument,such as clock delay and/or clock skew. Next, coefficients for the LTP 70that correct for the artificially introduced error condition aregenerated and stored in the LUT 72 and related to the particular errorcondition. Then, this cycle repeats with a new error conditionartificially introduced into the instrument and another set ofcoefficients for the LTP 70 are generated and stored in the LUT 72 andrelated to new error condition. This process is repeated as many timesas desired, depending on the size of the LUT 72 desired.

In one embodiment, entries to the LUT 72 are stored based on twomeasurements, clock delay and clock skew. In such an embodiment the LUT72 stores various sets of coefficients in a two-dimensional arrayselected by particular clock delay and skew values. Since clock delayand skew may be independent from one another, it is possible that aparticular delay value have multiple different skew values associated toit. Also, the converse is true, where a particular skew value may havecoefficients for several different delay values. FIG. 17 illustrates atwo-dimensional array storing coefficient values. Various coefficients,such as Coefficient Set 1, Coefficient Set 12, etc. are stored inindexed positions in the two-dimensional array. As described in moredetail below, at run time the instrument may measure clock delay andclock skew, and then use the measured values as indices to the LUT toselect a particular Coefficient Set. For example, if the delay value Dis measured along with clock skew value B, then Coefficient Set 14 isselected from the table. As described above, the coefficient set values,in this example those identified as Coefficient Set 14, which werepre-calculated to compensate for that particular combination of delayand clock skew, are then stored in the LTP 70. In this way theinstrument is compensated for the environmental run time conditions.

Clock delay and clock skew values may be positive or negative values.For example, clock delay values “A” and “B” may be negative clock delay,while clock delay values “C” and “D” may be positive clock delay.

Although illustrated as a two-dimensional array, concepts of theinvention extend to any number of index values for particular measuredvalues. Also, the two-dimensional array, such as that illustrated inFIG. 17, may additionally include a particular set of coefficientsindexed at zero clock delay and zero skew, since it is possible that theinstrument stay in or near factory calibration. In one embodiment theLUT 72 includes five different values for measured clock delay and threedifferent values for measured skew, including a central value for zeroclock delay and zero skew.

Some embodiments may select the particular coefficient set from the LUT72 that is closest to the intersection of the measured clock delay andclock skew. Yet other embodiments may use interpolation to generateparticular coefficient values that are not exactly indexed. For example,if the clock delay is measured as “A”, but the measured skew value fallsbetween values “C” and “D”, embodiments may interpolate a set ofcoefficient values that are “between” Coefficient Set 3 and CoefficientSet 4. Other techniques may also be used to generate coefficients, suchas two-dimensional linear interpolation, spline fitting, or otherinterpolation methods to approximate LTP 70 filter coefficients at theactual measured delay and skew values. Such interpolation allows one tocalibrate and store fewer LTP filter coefficient sets in thetwo-dimensional array LUT 72 for a given level of accuracy, savingfactory calibration time.

A particular method to populate the LUT 72 may occur as follows. The LTPfilter 70 is calibrated multiple times in the factory. Initially the LTPis calibrated having zero clock delay and zero skew. A nominal set ofcoefficients are generated for the LUT 72 and stored as the defaultcoefficients. Then, to generate the various coefficient values, theclock phase and skew are intentionally adjusted to be non-zero. Forexample, one ADC channel is adjusted to sample at a time of(delay+skew/2) and the other ADC channel is adjusted to sample at a timeof (delay−skew/2). Then coefficients are generated for the LTP filter 70to compensate for the combination of delay and skew. This process isrepeated at other clock delay values and clock skew values until the LUT72 is completely populated for conditions likely to be measured in thefield. After calibrating and storing LTP filter 70 coefficients in thistwo-dimensional array, such as the LUT 72, the hardware clock phasecontrols are returned to their nominal values (delay=skew=0) in a final,factory calibration. At run time, after an acquisition, the calibrationoscillator 300 can then be used to measure the actual clock phases,which may have drifted away from their nominal values, calculate theeffective delay and skew, and use these values as indices into thetwo-dimensional array of calibrated LTP filter coefficients, to find theLTP filter coefficients appropriate for the measured clock delay andskew. Then the selected coefficients are selected from the LUT 72 andstored in the LTP filter 70 for proper operation.

In general, compensation is performed after a signal has been acquiredby the instrument. For example, the signal is acquired and stored inmemory, such as the memory 400, 402 of FIG. 16. Then the compensationoscillator 300 is used to measure clock delay and clock skew. Next theclock delay and clock skew values are used as indices to the LUT 72 andparticular coefficients loaded into the LTP filter 70. Then theprocessing continues and the previously stored signal processed throughthe remainder of the channel, including the LTP filter 70.

Although compensation is typically performed after signal acquisition,it may also be performed before acquisition. Compensation may also beperformed at every signal acquisition, or merely periodically.Compensation may be performed at set intervals. In some embodimentsmultiple signals may be acquired sequentially and then a singlecompensation operation is performed and applied to all of the acquiredsignals. Such compensation may occur before the signals are acquired, orpreferably, after.

In cases of frequent acquisitions, there may be insignificant drift inthe hardware clock phases from one acquisition to the next. To savetime, software may measure the clock phases after each acquisition, butskip the interpolation step and reuse the previous LTP Filter 70coefficients if the delay and skew measurements were substantially thesame as before. This method saves delay introduced by loading thecoefficients, as well as any delay introduce by the interpolationtechniques described above, if used. Alternatively, if a user requestsfast and frequent acquisitions, for instance when using a fast-framemode, software may bypass use of the calibration oscillator betweenacquisitions altogether, instead collecting just one calibration burstat the end of the sequence, and using this one burst to determine theLTP coefficients to use for all frames within the sequence.

Although the above embodiments have described updating coefficients tothe LTP Filter 70, it is also possible to at least partially compensatefor run-time environmental changes by modifying the clocks that drivethe digital mixers, such as mixers 46, 52 of FIGS. 15 and 16. Asillustrated in FIGS. 15 and 16, a time adjust circuit 76 is coupled tothe mixer 46, while a time adjust circuit 74 is coupled to the mixer 52.Adjusting the time adjusters 74, 76 can partially or fully compensatefor the clock delay and clock skew, without needing to update thecoefficients for the LTP filter 70. In other embodiments it may also beadvantageous to combine the technique of updating the phases of thedigital mixing functions by using the time adjusters 74, 76, to avoidsignal amplitude loss or phase shift in the cross-over region, based onmeasured ADC channel clock delay with the technique described above toselect new coefficients for the LTP filter 70, to minimize interleavespurs, based on measured ADC channel clock skew and delay. If the twoapproaches are used together at run time, they may also be used togetherat factory calibration. In other words, when calibrating the multiplecoefficients for the LTP filter 70 in the two-dimensional array, thedigital mixing function phases may likewise be set based on measured ADCchannel clock delay in the same manner they will be set during regularacquisitions.

Although particular embodiments have been described, it will beappreciated that the principles of the invention are not limited tothose embodiments. Variations and modifications may be made withoutdeparting from the principles of the invention as set forth in thefollowing claims. For example, it is anticipated that a re-ordering ofthe digital filtering, mixing, and/or combining may allow for moreefficient execution of the digital processing while still providing forreconstruction of a digital representation of the input signal.

What is claimed is:
 1. A system for digitizing an input signal,comprising: an asynchronous time-interleaved (ATI) digitizer having aninput and including an analog harmonic mixer and a digital harmonicmixer; a compensation oscillator producing a compensation signalsuitable for inputting to the ATI digitizer; a switch configured toswitch the input of the ATI digitizer between the input signal and thecompensation signal; and a configurable filter, wherein the filter isconfigured by selecting a filter coefficient set from at least twostored pre-determined filter coefficient sets, and wherein the filtercoefficient set is selected based on a measurement made using thecompensation signal.
 2. The system of claim 1 wherein the analogharmonic mixer of the ATI digitizer, the compensation oscillator, andthe switch are included within an integrated circuit.
 3. The system ofclaim 2 wherein the compensation oscillator is tuneable over a frequencyrange wider than a frequency process uncertainty of the integratedcircuit.
 4. The system of claim 1 wherein the at least twopre-determined filter coefficient sets are stored in a Look Up Table(LUT).
 5. The system of claim 1, further comprising a time adjustcircuit coupled to the digital harmonic mixer of the ATI digitizer.
 6. Atest and measurement instrument, comprising: an input port for receivingan input signal from a device under test (DUT); a compensation signalgenerator for generating a compensation signal; a switch coupled to theinput port and to the compensation signal generator; an asynchronoustime-interleaved (ATI) digitizer coupled to the output of the switch,the ATI digitizer including a splitter, an analog mixing portion coupledto the splitter, a digitizing portion coupled to the analog mixingportion, a digital filtering portion coupled to the digitizing portion,a digital mixing portion coupled to the filtering portion, and acombiner coupled to the digital mixing portion; and a processorconfigured to perform a compensation procedure, by causing the switch tocouple the compensation signal to the ATI digitizer, determining asignal reconstruction error of the ATI digitizer by measuring the outputof the ATI digitizer in response to the compensation signal, andaltering operation of the ATI digitizer to remove the signalreconstruction error.
 7. The test and measurement instrument of claim 6,wherein the processor is further configured to determine a signalreconstruction error of the ATI digitizer by determining a phase errorvalue.
 8. The test and measurement instrument of claim 7, wherein thecompensation signal has an original frequency component, and atwice-converted frequency component resulting from passing through theATI digitizer, and wherein the processor is further configured todetermine a phase error value by comparing the original frequencycomponent to the twice-converted frequency component.
 9. The test andmeasurement instrument of claim 7, wherein the processor is furtherconfigured to alter operation of the ATI digitizer by adjusting a mixingfunction of a mixer in the analog mixing portion of the ATI digitizer.10. The test and measurement instrument of claim 7, wherein theprocessor is further configured to alter operation of the ATI digitizerby adjusting a mixing function of a mixer in the digital mixing portionof the ATI digitizer.
 11. The test and measurement instrument of claim7, wherein the processor is further configured to alter operation of theATI digitizer by adjusting a delay of a filter in the digital filteringportion of the ATI digitizer.
 12. The test and measurement instrument ofclaim 6, wherein the processor is further configured to perform acompensation procedure immediately after the test and measurementinstrument acquires the input signal from the DUT.
 13. The test andmeasurement instrument of claim 12, wherein the ATI digitizer furtherincludes a memory coupled to the output of the digitizing portion forstoring a digitized mixed signal.
 14. The test and measurementinstrument of claim 13, wherein the processor is further configured tobe triggered to automatically perform a compensation procedure when thedigitized mixed signal is stored in the memory.
 15. The test andmeasurement instrument of claim 6, wherein the processor is furtherconfigured to perform a compensation procedure in response to a commandreceived from a user interface.
 16. The test and measurement instrumentof claim 6, wherein the processor is further configured to perform acompensation procedure in response to detecting a trigger event.
 17. Atest and measurement instrument, comprising: an asynchronoustime-interleaved (ATI) digitizer including an input and producing anoutput signal; means for providing a known compensation signal to theinput of the ATI digitizer; means for measuring the output signal of theATI digitizer in response to the known compensation signal; means foradjusting a parameter of the ATI digitizer to minimize magnitude andphase differences between the output signal of the ATI digitizer and theknown compensation signal.
 18. The test and measurement instrument ofclaim 17, wherein the means for providing a known compensation signal tothe input of the ATI digitizer includes means for automaticallyde-coupling an input signal from a device under test (DUT) from theinput of the ATI digitizer.
 19. The test and measurement instrument ofclaim 17, wherein the means for measuring the output signal of the ATIdigitizer in response to the known compensation signal includes meansfor determining a phase error produced by the ATI digitizer.
 20. Thetest and measurement instrument of claim 17, wherein the means foradjusting a parameter of the ATI digitizer includes means for adjustinga mixing function of the ATI digitizer.